The use of green sand molds in foundry casting is well known. While green sand molds are relatively inexpensive to use (as compared with dry sand molds), their moisture content creates other problems which limit their usefulness, particularly in the casting of relatively intricate designs. For example, contact of molten metal with the green sand mold surface can vaporize the water and other organic materials to volatiles (steam, carbon oxides and the like). The steam can cause the mold to expand and the escaping volatiles can disrupt the mold surface such that the sand "burns in" the casting. These disruptions detrimentally affect the dimensional quality of the casting so as to preclude the commercial utilization of green sand molds for intricate designs.
It has therefore been proposed to utilize a facing sand composition which is used to form the molten metal-containing sand surface. The facing sand composition generally contains a major amount of sand substantially finer in particle size than the regular (system) sand. For example, system sand generally has a particle size of about 65 to 100 GFN (Grain Fineness Number as determined by the standards of the American Foundrymen's Society) while the sand used in a facing sand composition typically has a particle size of about 225 to about 250 GFN. In addition, the typical facing sand composition contains fine particles of a clay material along with cereal and/or other binders. In many areas of the country, there exists large deposits of bank sand which comprises a mixture of fine particles of sand and clay.
While the use of such facing sand compositions has improved the applicability of green sand molds to casting of intricate designs, problems remain. The reject rate of finished castings due to sand defects remains high. Casting surfaces are often pitted and dirty. In addition, many facing sand compositions have components which produce smoke which present health and/or safety hazards. Facing sand compositions are typically stable only for relatively short periods of time (e.g., about 1 day or less) and are often only moderately (or less) compatible with the system sand forming the bulk of the green sand mold.
The use of facing sand has also created a permeability problem in system sand. In commercial foundries, the sand used for a casting is recovered and recycled for later use in the production of other castings. The inclusion of facing sand (which includes fine sand and clay particles) in a mold thus results in the ultimate inclusion (by recycle) of facing sand in the system sand. Since the fine sand and clay particles of facing sand are inherently less permeable in a mold than the larger-sized sand particles of the system sand, the permeability of the system sand is eventually substantially reduced. The decreased permeability of the system sand results in increased problems in the use of such sand for large castings (e.g., about 8 pounds or more) which generate relatively large amounts of volatiles. For example, the decreased permeability can result in mold wall movement, swells and thermal cracking, any or all of which can result in unacceptable castings. Generally, these large castings do not utilize facing sand per se.